1. Field of the Invention
The present invention relates generally to coring of subterranean formations, and more specifically to an improved method and apparatus for pressure coring using a non-invading gel to coat and protect the core as it is cut, enters the core barrel, and is retrieved to the surface for analysis.
2. State of the Art
Pressure coring involves recovering a core cut from subterranean formations under downhole ambient temperature and pressure conditions at the coring target site, and retrieving the core to the surface in a pressure-tight core barrel compartment under pressure approximating that at which the core was cut. This is in contrast to a normal coring operation, where the core is cut and retrieved, and during the trip to the surface gases and liquids present within the core tend to bleed out of the core sample as the core barrel is not pressure-tight and the ambient pressure surrounding the core sample decreases with decreasing depth of the borehole.
In order for the data obtained from core to have significance to reservoir engineers, the core to be analyzed must be representative of the reservoir rock. Changes in environmental conditions between the coring target site and the laboratory in which the core is analyzed tend to alter the fluid content and saturation values of a core. Hence, the pressure core barrel was developed to recover cores under pressurized conditions by containing the core in a pressure-sealed chamber or compartment prior to retrieving the core to the surface. Ideally, a pressure core barrel provides the capability to retrieve cores at a controlled pressure in substantial isolation from changes in ambient temperature and pressure and free from gas and liquid leakage.
The first notably successful pressure core barrel was developed by the Esso Production Research Co. in the late 1960's and is described in U.S. Pat. No. 3,548,958 issued to Blackwell et al., the disclosure of which is hereby incorporated by this reference. The device of the '958 patent employed a self-contained high pressure nitrogen gas supply valved through a regulator controlled by expansion of an accumulator compartment to maintain approximate formation pressure of the core sample after it was cut and trapped in the pressure-tight compartment of the barrel.
An improved pressure core barrel operating along generally the same principles as that of the '958 patent was developed by Christensen, Inc., a predecessor company to the assignee of the present invention. The Christensen pressure core barrel and method of operation is described in U.S. Pat. Nos. 4,256,192 and 4,272,987, the disclosures of each being incorporated herein by reference. Drawings from the '987 patent and much of the text describing the preferred embodiment of that core barrel have been employed in the detailed description of the preferred embodiment of the present invention.
While pressure coring has proven to be a valuable tool for formation evaluation, in recent years it has fallen into disuse for several reasons. First, post-coring handling of the core barrel and core as described in the '987 patent, wherein flushing of the drilling mud from the core barrel and freezing of the core are performed, adds complexity to the operation and may be difficult to perform at offshore and other remote sites. Second, drilling fluid invasion of the formation ahead of the bit as well as contamination of the core during the subsequent flushing operation may degrade core quality.
In addition to some deficiencies in pressure coring techniques, improvements in formation testing techniques such as reservoir flow testing and drill stem testing led to a certain degree to a de-emphasis on coring. Reservoir flow testing is employed in producing reservoirs to alternately flow and shut in a producing formation to determine its recovery characteristics and production potential. Drill stem tests also involve the alternate flowing and shutting in of a producing formation and measuring the pressure recovery after the formation is flowed during the shut-in periods. Drill stem tests are traditionally shorter in duration than reservoir flow tests, and most often undertaken during completion of a new well. Many times the formation is initially perforated with the testing string in the borehole, the most popular such operation in recent years being a so-called tubing conveyed perforating operation, where the perforating gun is run into the borehole on the same string as, and below, the testing tools and packer. Such operations, while effective for many purposes, are expensive, entail some substantial risk to personnel, equipment and the formation itself, and do not recover either a sample of producing formation rock or a sample of formation fluids in a relatively pristine condition as they naturally subsist within the formation. Stated another way, the fluids flowing out of the formation which may be trapped in the testing string are not necessarily representative of the fluids in situ in the formation.
Non-pressurized coring can recover formation rock core samples, but the mechanical and chemical properties as well as the fluid contents of such core samples are different from when the core is taken, due to fluids bleeding of the sample under decreasing pressure as the core sample is withdrawn from the borehole. Further, pressure reduction as the core sample is withdrawn results in precipitation of heavy components, such as asphaltene, of the crude oil contained in the sample.
Thus, pressure coring would seem to be an ideal technique for evaluation of the interrelated rock and fluid characteristics of a producing formation, but for certain specific disadvantages including core invasion and post-coring handling. Flushing of the core barrel, either on the surface as described in U.S. Pat. No. 4,272,987 or downhole as described in U.S. Pat. No. 5,356,872, while meritorious in concept, may result in contamination of the core from the flushing fluid by intrusion into the pores in the core samples, displacing natural formation fluids and altering the chemical and mechanical characteristics of the formation rock. Since the core sample may already be of poor quality due to formation invasion by drilling fluid ahead of the bit, operators in many instances have been reluctant to heavily rely upon coring results when making decisions.
The other major disadvantage of state of the art coring techniques, as noted above, is the requirement of freezing the core by packing it in dry ice prior to bleeding off gas pressure, and then maintaining the core sample in a frozen state during transport to a laboratory. This practice may alter the physical and chemical integrity of the core, as well as requiring extra materials, equipment and labor.
It would be highly beneficial to provide a pressure coring system which also afforded the opportunity to cut and preserve the core sample in a relatively pristine state so as to provide a combination of formation rock and fluid for analysis, particularly if the post-coring core handling operation could be conducted in a simple, straightforward manner without detrimental results. However, to date, no state of the art technique offers such advantages.